Delay locked loop synthesizer with multiple outputs and digital modulation

Abstract

A delay locked loop circuit ( 200 ) in which multiple outputs are produced. A single delay line ( 24 ) is shared among multiple tap selection circuits ( 256 A, 265 B, 265 C). Fixed phase shifts ( 412 ) can be introduced between multiple outputs. A modulating signal can be used in the tap selection processing to produce digital amplitude, frequency and/or phase modulation.

1. A circuit, comprising:
 a delay locked loop having a delay line with a plurality of tap outputs; 
 a first tap selection circuit that produces a first set of tap addresses to select a first set of the plurality of tap outputs from the delay line according to a first timing to produce a first output signal; 
 a second tap selection circuit that produces a second set of tap addresses to select a second set of the plurality of tap outputs from the delay line according to a second timing to produce a second output signal; 
 a modulator combining the first and second output signals to produce a modulated output signal; and 
 
       wherein the modulator frequency-modulates the first output signal with the second output signal. 
     
     
       2. A circuit, comprising:
 a delay locked loop having a delay line with a plurality of tap outputs; 
 a first tap selection circuit that produces a first set of tap addresses to select a first set of the plurality of tap outputs from the delay line according to a first timing to produce a first output signal; 
 a second tap selection circuit that produces a second set of tap addresses to select a second set of the plurality of tap outputs from the delay line according to a second timing to produce a second output signal; 
 a modulator combining the first and second output signals to produce a modulated output signal; and 
 wherein the modulator phase-modulates the first output signal with the second output signal. 
 
     
     
       3. A circuit, comprising:
 a delay locked loop having a delay line with a plurality of tap outputs; 
 a first tap selection circuit that produces a first set of tap addresses to select a first set of the plurality of tap outputs from the delay line according to a first timing to produce a first output signal; 
 a second tap selection circuit that produces a second set of tap addresses to select a second set of the plurality of tap outputs from the delay line according to a second timing to produce a second output signal; 
 a modulator combining the first and second output signals to produce a modulated output signal; and 
 
       wherein the modulator amplitude-modulates the first output signal with the second output signal. 
     
     
       4. A circuit, comprising:
 a delay locked loop having a delay line with a plurality of tap outputs; 
 a first tap selection circuit that produces a first set of tap addresses to select a first set of the plurality of tap outputs from the delay line according to a first timing to produce a first output signal; 
 a second tap selection circuit that produces a second set of tap addresses to select a second set of the plurality of tap outputs from the delay line according to a second timing to produce a second output signal; 
 a tap selection processor that selects the first set of the plurality of tap outputs from the delay line according to the first timing, and selects the second set of the plurality of tap outputs from the delay line according to the second timing, the tap selection processor comprising:
 a frequency accumulator receiving an integer part K of K.C where K.C=F out /F ref , where F out  is a desired output frequency and F ref  is a reference clock frequency, and wherein the frequency accumulator is clocked by F ref ; and 
 a phase accumulator that receives the fractional part C of K.C, wherein the phase accumulator is clocked by an overflow of the frequency accumulator, and wherein the frequency accumulator produces the first set of the plurality of tap output addresses as an output thereof; 
 
 a first demultiplexer responsive to the tap selection processor to selectively route the selected first set of the plurality of tap outputs to a common node to produce the first output signal; and 
 a second demultiplexer responsive to the tap selection processor to selectively route the selected second set of the plurality of selected tap outputs to another common node to produce the second output signal. 
 
     
     
       5. A circuit, comprising:
 a delay locked loop having a delay line with a plurality of tap outputs; 
 a first tap selection circuit that produces a first set of tap addresses to select a first set of the plurality of tap outputs from the delay line according to a first timing to produce a first output signal, the first tap selection circuit further comprising:
 a first tap selection processor that selects the first set of the plurality of tap outputs from the delay line according to the first timing; and 
 a first demultiplexer responsive to the first tap selection processor to selectively route the selected first set of tap outputs to a common node to produce the first output signal; 
 
 a second tap selection circuit that produces a second set of tap addresses to select a second set of the plurality of tap outputs from the delay line according to a second timing to produce a second output signal, the second tap selection circuit further comprising:
 a second tap selection processor that selects the second set of the plurality of tap outputs from the delay line according to the second timing; and 
 a second demultiplexer responsive to the second tap selection processor to selectively route the selected second set of tap outputs to another common node to produce the second output signal; 
 
 
       and wherein the first tap selection processor comprises:
 a frequency accumulator receiving an integer part K of K.C where K.C=F out /F ref , where F out  is a desired output frequency and F ref  is a reference clock frequency, and wherein the frequency accumulator is clocked by F ref ; and 
 a phase accumulator that receives the fractional part C of K.C, wherein the phase accumulator is clocked by an overflow of the frequency accumulator, and wherein the frequency accumulator produces the first set of the plurality of tap output addresses as an output thereof. 
 
     
     
       6. The apparatus according to  claim 5 , wherein the second tap selection processor comprises a phase offset adder receiving the first set of the plurality of tap outputs as a first input and a normalized phase shift as a second input thereto and producing the second set of the plurality of tap output addresses as an output thereof, and wherein the phase offset adder is clocked by the overflow of the frequency accumulator. 
     
     
       7. A circuit for producing two output signals having frequency F out  and differing by a phase shift, comprising:
 a delay locked loop having a plurality of addressable tap outputs; 
 a tap selection circuit that selects a first sequence of tap addresses C ja  from the addressable tap outputs; 
 an adder that adds a normalized phase shift component Φ=α/ (2 π) (α=a desired phase shift in radians) to the first sequence of tap addresses C ja  to produce a second sequence of tap addresses C jb , where C is the fractional part of K.C in K.C=F out /F ref,  and F ref  being a reference clock frequency; and 
 a first multiplexer and a second multiplexer, wherein the first sequence of tap addresses C ja  are applied to the first multiplexer to produce a first output signal F outa , and wherein the second sequence of tap addresses C jb  are applied to the second multiplexer to produce a second output signal F outb , and wherein F outa  differs from F outb  by the desired phase shift. 
 
     
     
       8. The apparatus according to  claim 7 , wherein the desired phase shift comprises ±90 degrees. 
     
     
       9. The apparatus according to  claim 7 , wherein the desired phase shift component Φ comprises a time varying phase shift component. 
     
     
       10. A circuit for producing two output signals differing by a phase shift, comprising:
 a delay locked loop having a plurality of addressable delay line tap outputs, the delay locked loop synthesizing the output signals at a frequency F out , with K.C=F out /F ref , and F ref  being a reference clock frequency; 
 a tap selection circuit that selects a sequence of tap addresses C ja  from the plurality of addressable delay line tap outputs; 
 a first multiplexer, wherein the sequence of tap addresses C ja  are applied to a plurality of inputs of the first multiplexer to produce a first output signal F outa ; 
 a second multiplexer, wherein the sequence of tap addresses C ja  are added to a delay factor α/(2π) where α is a desired phase shift in radians and applied to a plurality of inputs of the second multiplexer to produce a second output signal F outb ; and 
 
       wherein the first and second multiplexers each comprise N:1 multiplexers having N inputs. 
     
     
       11. The apparatus according to  claim 10 , wherein α corresponds to a fixed phase shift. 
     
     
       12. The apparatus according to  claim 10 , wherein the α corresponds to ±90 degrees. 
     
     
       13. The apparatus according to  claim 10 , wherein the α comprises a time varying phase shift. 
     
     
       14. A digital frequency modulator, comprising:
 a delay locked loop having a delay line with a plurality of tap outputs; 
 a tap selection processor that selects, from the delay line with the plurality of tap outputs, a sequence of time varying tap addresses C j (t) that vary in time in accordance with a modulating signal m(t); and 
 a multiplexer circuit, and wherein the time varying tap addresses C j (t) are applied to the multiplexer circuit to select a time varying sequence of tap outputs as a frequency modulated output signal F out (t) wherein the tap selection processor comprises an integrator that integrates the modulating signal m(t) and an adder that adds the integrated modulating signal m(t) to a selected sequence of tap addresses C j  to produce C j (t). 
 
     
     
       15. The apparatus according to  claim 14 , wherein the time varying tap addresses C j (t) are further selected to amplitude modulate the output signal F out (t) in accordance with the modulating signal m(t). 
     
     
       16. The apparatus according to  claim 14 , wherein the time varying tap addresses C(t) are further selected to phase modulate the output signal F out (t) in accordance with the modulating signal m(t). 
     
     
       17. The apparatus according to  claim 14 , wherein the modulating signal m(t) comprises an analog signal. 
     
     
       18. The apparatus according to  claim 14 , wherein the modulating signal m(t) comprises a data signal. 
     
     
       19. A digital amplitude modulator, comprising:
 a delay locked loop having a delay line with a plurality of tap outputs; 
 a tap selection processor that selects, from the delay line with the plurality of tap outputs, a sequence of time varying tap addresses C j (t) that vary in time in accordance with a modulating signal m(t); 
 a multiplexer circuit, and wherein the time varying tap addresses C j (t) are applied to the multiplexer circuit to select a time varying sequence of tap outputs as an output signal F out (t); and 
 
       wherein the tap selection processor comprises:
 a first adder that adds the modulating signal m(t) to a selected sequence of tap addresses C ja  to produce a first sequence of time varying tap addresses C jb (t); 
 a second adder that subtracts the modulating signal m(t) from the selected sequence of tap addresses C ja  to produce a second sequence of time varying tap addresses C jc (t); and 
 wherein C j (t) comprises C jb (t) and C jc (t); and wherein the multiplexer circuit comprises: 
 a first multiplexer receiving the first sequence of time varying tap addresses C jb (t) to produce a first output signal V 1 (t); and 
 a second multiplexer receiving the second sequence of time varying tap addresses C jc (t) to produce a second output signal V 2 (t); 
 
       and further comprising:
   a summation circuit that adds V 1 (t) to V 2 (t) to obtain an amplitude modulated output signal V(t).   
 
     
     
       20. The apparatus according to  claim 19 , wherein the adding circuit comprises an analog summation circuit. 
     
     
       21. The apparatus according to  claim 20 , wherein the summation circuit comprises a three state analog summation circuit. 
     
     
       22. The apparatus according to  claim 21 , wherein the three state analog summation circuit comprises a NAND gate and a NOR gate, each receiving V1(t) and V2(t) and producing first and second outputs therefrom; and a first and a second transistor connected in series and driven by the first and second outputs respectively to produce a three state output at a node coupling the first transistor with the second transistor. 
     
     
       23. The apparatus according to  claim 19 , wherein the time varying tap addresses C j (t) are further selected to frequency modulate the output signal F out (t) in accordance with the modulating signal m(t). 
     
     
       24. The apparatus according to  claim 19 , wherein the time varying tap addresses C(t) are further selected to phase modulate the output signal F out (t) in accordance with the modulating signal m(t). 
     
     
       25. The apparatus according to  claim 19 , wherein the modulating signal m(t) comprises an analog signal. 
     
     
       26. The apparatus according to  claim 19 , wherein the modulating signal m(t) comprises a data signal. 
     
     
       27. A method of producing multiple output frequencies using a delay locked loop having a delay line with a plurality of tap outputs, comprising:
 selecting a first sequence of the tap outputs from the delay line having the plurality of tap outputs according to a first timing to produce a first output signal F out1 ; 
 selecting a second sequence of the tap outputs from the delay line having the plurality of tap outputs according to a second timing to produce a second output signal F out2 .; and 
 applying addresses of the first sequence of tap outputs to a first multiplexer to perform selecting the first sequence of tap outputs; and 
 applying addresses of the second sequence of tap outputs to a second multiplexer to perform selecting-the second sequence of tap outputs; and 
 
       modulating the first sequence of tap outputs with the second sequence of tap outputs. 
     
     
       28. A method of producing two output signals differing by a phase shift in a delay locked loop circuit having a delay line with a plurality of addressable tap outputs, comprising:
 selecting a first sequence of tap addresses C ja ; 
 adding a phase shift component Φ to the first sequence of tap addresses C ja  to produce a second sequence of tap addresses C jb ; and 
 applying the first sequence of tap addresses C ja  to a first multiplexer to produce a first output signal F outa ; and 
 applying the second sequence of tap addresses C jb  to a second multiplexer to produce a second output signal F outb , and wherein F outa  differs from F outb  by a phase shift related to Φ. 
 
     
     
       29. The method according to  claim 28 , wherein the phase shift comprises ±90 degrees. 
     
     
       30. The method according to  claim 28 , wherein the phase shift component Φ comprises a time varying phase shift component. 
     
     
       31. A method of producing two output signals differing by a phase shift using a delay locked loop having a plurality of addressable delay line tap outputs, comprising:
 selecting a sequence of tap addresses C ja  from the delay locked loop having a delay line providing the plurality of addressable delay line tap outputs; 
 applying the sequence of tap addresses C ja  to a plurality of inputs of a first multiplexer to produce a first output signal F outa ; and 
 applying the sequence of tap addresses C ja  to a plurality of inputs of a second multiplexer to produce a second output signal F outb ; 
 wherein F out1  differs from F out2  by the phase shift, and wherein the phase shift is determined by a constant difference in address location selected by the sequence of tap addresses C ja  between the first and second multiplexers. 
 
     
     
       32. The method according to  claim 31 , wherein the first and second multiplexers are connected to the tap outputs in a manner such that a tap address from the sequence of tap addresses selects an input corresponding to tap number L of the first multiplexer and corresponding to tap number L+Φ in the second multiplexer, where Φ is a phase shift normalized to a length of the delay line. 
     
     
       33. The method according to  claim 32 , wherein the sequence of tap addresses C ja  is applied to the second multiplexer through a delay element. 
     
     
       34. The apparatus according to  claim 31 , wherein the phase shift corresponds to ±90 degrees. 
     
     
       35. The apparatus according to  claim 31 , wherein the phase shift comprises a time varying phase shift. 
     
     
       36. A method of providing digital frequency modulation in a delay locked loop circuit having a delay line with a plurality of tap outputs, comprising:
 selecting a sequence of time varying tap addresses C j (t) that vary in time in accordance with a modulating signal m(t), wherein the selecting comprises:
 integrating the modulating signal m(t); and 
 adding the integrated modulating signal m(t) to a selected sequence of tap addresses C j  to produce C j (t); and 
 
 applying the time varying tap addresses C j (t) to a multiplexer circuit to select a time varying sequence of tap outputs as a frequency modulated output signal F out (t). 
 
     
     
       37. A method of providing digital amplitude modulation in a delay locked loop circuit having a delay line with a plurality of tap outputs, comprising:
 selecting a sequence of time varying tap addresses C j (t) that vary in time in accordance with a modulating signal m(t), wherein the selecting comprises:
 adding the modulating signal m(t) to a selected sequence of tap addresses C ja  to produce a first sequence of time varying tap addresses C jb (t); and 
 subtracting the modulating signal m(t) from the selected sequence of tap addresses C ja  to produce a second sequence of time varying tap addresses C jc (t), 
 wherein C j (t) comprises C jb (t) and C jc (t); and 
 
 applying the time varying tap addresses C j (t) to a multiplexer circuit to select a time varying sequence of tap outputs as an amplitude modulated output signal F out (t). 
 
     
     
       38. The method according to  claim 37 , and wherein the applying comprises:
 applying the first sequence of time varying tap addresses C jb (t) to a first multiplexer to produce a first output signal V 1 (t); and 
 applying the second sequence of time varying tap addresses C jc (t) to a second multiplexer produce a second output signal V 2 (t). 
 
     
     
       39. The method according to  claim 38 , further comprising adding V 1 (t) to V 2 (t) to obtain an amplitude modulated output signal V(t). 
     
     
       40. The method according to  claim 39 , wherein the adding is carried out in an analog summation circuit. 
     
     
       41. The method according to  claim 40 , wherein the analog summation circuit comprises a three state analog summation circuit. 
     
     
       42. A method of selecting delay line taps to produce an output signal from a delay locked loop, comprising:
 computing an tap address P.Q where P is an integer part and Q is a fractional part; and 
 selecting a delay line tap address of P during a portion of an operational cycle and of P+1 during a remainder of the operational cycle, with the regularity of selection of P and P+1 determined by an algorithm that establishes an average value of the tap address as approximately P.Q. 
 
     
     
       43. The method according to  claim 42 , wherein the algorithm selects the value of P for 1-0.Q operational cycles and selects the value of P+1 for 0.Q operational cycles.